An x-ray free electron laser oscillator (XFELO) is a next-generation hard x-ray source, which will produce fully coherent x-ray beams of record spectral purity and average brightness in the photon energy range from about 5 to 25 keV. The characteristics of an XFELO are complementary to those of self-amplified spontaneous emission (SASE) and are briefly summarized below. The goal of the workshop is to explore the scientific opportunities that such an x-ray source could open up, and thus to evaluate an XFELO as a future light source complementary to the upgraded APS.

XFELO Characteristics

Photon energy coverage from 5 keV to 25 keV (and third harmonics)

Tunable photon energy (5%)

Fully coherent transversely and temporally

High spectral purity with ~1 meV bandwidth

Length of individual x-ray pulse ~0.1-1 ps

Number of photons ~109 per pulse (~106–104 for 3rd harmonics)

Peak brightness comparable to that of SASE XFELs

Pulse repetition rate ~1 MHz

Time-averaged brightness is five orders of magnitude higher than that of the LCLS and three orders of magnitude higher than that of the European XFEL.